"Unicorn in a Field of Horses": A Rare Case of Carcinosarcomatous Transformation in a Uterine Leiomyoma

Malignant transformation in leiomyoma is rare, with a few documented cases of sarcomatous transformation in leiomyomas. However, carcinosarcomatous transformation in leiomyomas is extremely infrequent. A 45-year-old female presented with a mass sensation per abdomen for the last five months. An abdominal ultrasound revealed multiple uterine fibroids. Contrast-enhanced magnetic resonance imaging revealed a large multiloculated abdominopelvic mass arising from the fundus of the uterus, along with multiple smaller subserosal fibroids. A total abdominal hysterectomy with bilateral salpingectomy was performed. Grossly, the uterus was bosselated with multiple subserosal fibroids. The larger degenerated fibroid showed a smooth outer surface; however, the cut surface was predominantly cystic and filled with necrotic material. Microscopically and immunohistochemically, the larger mass showed a high-grade biphasic tumor comprising carcinomatous and sarcomatous components with the peripheral solid areas showing compressed smooth muscle bundles representing the residual leiomyomatous areas. A final diagnosis of carcinosarcomatous transformation in leiomyoma was rendered. The index report highlights the significance of systematic gross and microscopic examination of all the uterine masses in patients with multiple leiomyomata.